A typical implantable cardioverter defibrillator (ICD) has the capability of providing a variety of anti-tachycardia pacing (ATP) regimens as well as cardioversion/defibrillation shock therapy. Normally, arrhythmia therapies are applied according to a pre-programmed sequence of less aggressive to more aggressive therapies depending on the type of arrhythmia detected. Typically, termination of an arrhythmia is confirmed by a return to either a demand-paced rhythm or a sinus rhythm in which successive spontaneous R-waves are separated by at least a defined interval. When ATP attempts fail to terminate the tachycardia, high-voltage cardioversion shocks may be delivered. Since shocks can be painful to the patient and consume relatively greater battery energy than pacing pulses, it is desirable to avoid the need to deliver shocks by successfully terminating the tachycardia using less aggressive pacing therapies. On the other hand, if a tachycardia is a lethal arrhythmia that is likely to require a shock therapy for successful termination, it is desirable to deliver the shock therapy as quickly as possible without delay.
The success of a tachycardia therapy depends in part on the accuracy of the tachycardia detection. In some cases, a tachycardia originating in the atria, i.e. a supraventricular tachycardia (SVT), is difficult to distinguish from a tachycardia originating in the ventricles, i.e. a ventricular tachycardia (VT). For example, both the atrial chambers and the ventricular chambers may exhibit a similar tachycardia cycle length when an SVT is conducted to the ventricles or when a VT is conducted retrograde to the atria.
Accordingly, accurate classification of a detected tachycardia as VT or SVT is needed in order to properly determine when and what type of therapy is necessary. As more complex algorithms become available for accurately detecting and discriminating cardiac rhythms with a high sensitivity and high specificity, the processing time and burden on the ICD for performing these algorithms increases. These relatively more complex algorithms may be needed when the rhythm type is difficult to discern. At times, however, more complex algorithms may pose undue processing burden. What is needed, therefore, is a method and apparatus for discriminating SVT and VT with high sensitivity and specificity while limiting the signal processing burden.